Gene regulatory proteins such as repressors and activators are involved in a variety of fundamental cellular processes. These proteins bind specific DNA sequences and regulate the expression of a multitude of essential genes. A true understanding of the structure of these proteins at atomic resolution would allow the rational design of proteins that could bind to specific DNA sequences in cellular or viral DNA. In research described in this proposal, x-ray crystallography will be used to determine the three dimensional structures of bacteriophage P22 Mnt repressor and several Arc/Mnt hybrid proteins bound to mnt operator DNA. The N-terminal DNA binding domains of Mnt and the Arc/Mnt hybrids will be cloned, expressed, and purified. Protein-DNA Co-crystals will be grown and the three-dimensional structure of the complexes will be determine at atomic resolution. The structural information gained from these studies will undoubtedly enhance our understanding of how distinct operator sequences are recognized and how a single residue change can result in drastic changes in DNA binding specificity of a protein. This understanding will augment our ability to design proteins with novel DNA binding specificities for use in medicinal and research applications.